Mass Problems with propellant from the Moon

[RuBisCO]

There is lots of talk of returning to the the moon and mining its polar areas of water. Water can make propellant to bring cargo and crews back from the moon and propel a space economy, but specifics are not so good. You see Water needs to be converted to Liquid hydrogen and liquid oxygen, a rather energy intensive process. Lets say you have 1 MW thermal of power to work with, that could produce 100 tons of propellant at 10% efficency (electrolysis loses, coversion to electricity loses, power mining equipment, cryo coolers) in 155 days, or 235 tons a year. Half of that fuel is lost to get it to lunar orbit, only about 25% if we consider reusuable fuel shuttles from the surface and back, so that coming to ~56 tons of propellant in as far as Earth-Moon L2 per year. 

To provide that much power it is often assumed that requires nuclear reactors, its easily possible to do 1 kg/kw thermal, but the added electrical conversion equipment and radiators is going to bring that up to 20-40 kg/kw, so that comes to 40 ton power plant, landed on the moon. But wait that is not including the electolysis and cryoplant for making and liquifying the fuel.  

Solar panels can do at most 2 kg/kw, but at least that is as electricity, but that is not including stuctural mass which is going to be more on the moon then in zero gravity. Lets say 10 kg/kw and an efficency of 20% instead of 10% (so 500 kWe is needed instead of 1000 kWh) and that comes to 5 tons, not bad. At the poles of the moon there are places of "eternal sunlight" where direct sun light is available 80-90% of the time, so it may be possible with present day solar photovoltaics to buid multple megawatts worth of solar power farms at the moons poles. Lets say 30% efficent solar panels are used, hung from a mast a 35 m by 7 m array could produce 100 kw, this mind you is 2-3 times more efficient then the ones on the ISS. 

Add the mass for electrolysis and cyrocoolers and radiators, lots of radiators, we are likely talking about at least 100 tons of eqipment for at most 1 tons of fuel a day.  

[tater]

I have a paper from the 1990s someplace around here that shows that lunar ISRU basically just offsets landing costs (in propellant).

A better place for a propellant depot using ISRU is in an Earth Moon Lagrange point, and find a NEO object with water to drag there.

Honestly, as a few people have said since te 1960s, if you can reuse spacecraft, and massively reduce the cost to LEO, everything becomes much simpler.

235 tons a year in your example is not that many launches of something like Starship. If Starship works, there's no reason not to simply make a bigger one, either, if the need is for 200+ tons of propellant.

[Bill Phil]

I think it only really works well if you have electric launch capability to deliver payloads to lunar orbit from the lunar surface. 

One idea I’ve read about is to bring hydrogen from Earth and acquire oxygen from the lunar surface. Also energy intensive, but it’s not too bad. With electric launch infrastructure you could just deliver raw regolith that’s been compressed to a standard density. Then the oxygen can be removed from the regolith, and if we wanted to we could get metals and some other stuff from the regolith as well which could be beneficial for space infrastructure.

Of course you need an electric launcher on the Moon. And that’s not easy to do.

I wonder how well a slingatron would work on the Moon...?

[magnemoe]

1 hour ago, Bill Phil said:

I think it only really works well if you have electric launch capability to deliver payloads to lunar orbit from the lunar surface. 

One idea I’ve read about is to bring hydrogen from Earth and acquire oxygen from the lunar surface. Also energy intensive, but it’s not too bad. With electric launch infrastructure you could just deliver raw regolith that’s been compressed to a standard density. Then the oxygen can be removed from the regolith, and if we wanted to we could get metals and some other stuff from the regolith as well which could be beneficial for space infrastructure.

Of course you need an electric launcher on the Moon. And that’s not easy to do.

I wonder how well a slingatron would work on the Moon...?

ISRU to reduce the recurring launch and landing cost would work even if you need to bring in some fuel from earth it would way less than fuel for landing and return. 
Fuel for export pretty much require an electrical launcher on moon. 

[mikegarrison]

How much delta-v can you really get from green cheese anyway?

[tater]

1 hour ago, mikegarrison said:

How much delta-v can you really get from green cheese anyway?

Quite a bit, cheese is like 85% water

 

[kerbiloid]

11 minutes ago, tater said:

1 hour ago, mikegarrison said:

How much delta-v can you really get from green cheese anyway?

Quite a bit, cheese is like 85% water

It contains deuterium (as any water), so you can feed it to a fusion reactor.

[Nuke]

as a fuel depo a permanent installation seems like it would be a good investment, especially as part of a permanent lunar base. it probably wouldn't fuel a starship, but a reusable ssto lunar shuttle to go between the lunar surface and the lunar gateway that might provide enough fuel for fairly regular operation. maybe make that fuel go further by using arcjets, up your isp an extra 100-200s. if you can also manufacture fuel tanks on the moon you can go with a staged shuttle, or perhaps use drop tanks, but the energy requirements for that would be significantly greater than just making fuel. if you need more fuel, either upgrade the solar installation or add a nuclear power plant to the base. 

Edited July 13, 2019 by Nuke

[kerbiloid]

A lunar orbital power plant equipped with a microwave emitter.
An on-ground launch magnapult powered with this microwave.
A lunar shuttle equipped with cans of lunar dust to be ionized and electrically thrown back to produce thrust to orbit, deorbit, and land.

Edited July 13, 2019 by kerbiloid

[RuBisCO]

Another problem I found, and sort of solved: The oxidizer to fuel ratio problem

LH2/LO2 rocket engine does not work on 1:8 H2:O2 mass ratio that is in water, rather ideal ratio is somewhere between 1:6 and 1:5, these means we have excess oxygen to deal with.

It turns out though that lunar ice is more then just water, according to the LCROSS mission spectral data, for every 100 kg of water, there was 16.75 kg of SH2, 6.03 kg of NH3, 3.19 kg of SO2, 3.14 kg of C2H4, 2.17 kg of CO2, 1.55 kg CH3OH, .65 kg of CH4, in short for every 100 kg of water there is an extra >31.2 kg of volatilies, most of which have hydrogen. If we oxidize all these with the extra oxygen not needed for propellant we get back more hydrogen then we need, we end up with an oxygen deficient! The SH2 and SO2 we can burn and convert to sulfur soot and recover all the hydrogen and oxygen, the organics we could pyrolysis to carbon soot and get back all the oxygen and hydrogen. Only the nitrogen we would need  to convert to NO stored as a liquid in tanks in the permanent shadowed craters of the moon. Alternatively extra oxygen can be made by reducing lunar soil to metals, allowing us to make NO2 and then react with lunar soil to make solid nitrates that we can dump.

 

Edited July 14, 2019 by RuBisCO

[kerbiloid]

Spoiler

So, the lunar ice deposits are enough to research where were the lunar ice deposits spent for the research

 

[Guest]

On 7/13/2019 at 4:24 PM, Nuke said:

as a fuel depo a permanent installation seems like it would be a good investment, especially as part of a permanent lunar base. it probably wouldn't fuel a starship, but a reusable ssto lunar shuttle to go between the lunar surface and the lunar gateway that might provide enough fuel for fairly regular operation.

If you had a permanent base on the moon it seems like Gateway would be pretty irrelevant. Certainly not an important destination. The base/fuel depo would be the hub of activity imo.

[Rus-Evo]

Quick question. What does the lunar mining look like in practice.  Does a digger mine soggy soil which then has the water seperated by a machine? How big a hole in the ground is created to fuel, lets say one Starship?

[kerbiloid]

Land the Starship on the Moon, make the passengers get out, and give them pickaxes and shovels. The better they work, the sooner they fly.

As probably the famous lunar ice is in fact just rare traces of hydrates scattered in regolith, then it will be milling of the regolith into dust, heating it to hudreds °C to decompose the hydrates, then gather the water and run it through numerous filters and dystillers to purify it. So, many, many times more regolith than the Starship masses itself.

Edited July 14, 2019 by kerbiloid

[RuBisCO]

2 hours ago, Rus-Evo said:

Quick question. What does the lunar mining look like in practice.  Does a digger mine soggy soil which then has the water separated by a machine? How big a hole in the ground is created to fuel, lets say one Starship?

Well the architectures are all over the place in design, ULA thinks this: 

I'm thinking simpler: just have a hauling robot go down into the dark craters, dug up a few tons of frozen soil, put it in a enclosed tank, and haul it back up to base on a "peak of eternal sunlight" at the south pole, there a processing facility can do all the work of heating it, boiling out the water and other goodies, make stuff out of the left over dirt, etc, a kind of "use every part of the buffalo" philosophy. 

As for Starship, that needs methane, which means carbon, and there is over 1 order of magnitude less carbon (As CO2, CH4, C2H4, CH3OH, etc) frozen in the polar soil than water. Thus LCH4-LO2 fuel economy does not work without a much more dominate LH2-LO2 fuel economy utilizing the water first. If we suppose that everyone ton of soil has 5% ice, and for every ton of water we get 40 kg of carbon volatilies, of which we can make into 53 kg of methane at a cost of hydrogen from water, then for every ton of lunar soil we get 0.265 kg of methane, and starship takes at least 240,000 kg... so a very big hole. 

 

[kerbiloid]

1 hour ago, RuBisCO said:

ULA thinks this

As the water ice melts at 0°C and boils (in vacuum) at < 100°C, the lunar temperature daily variation is +/- 100°, and the youngest ice on the Moon is 4 bln years old, ULA plans look even more optimistic than such construction in Sahara.

Edited July 14, 2019 by kerbiloid

[Nuke]

5 hours ago, Dale Christopher said:

If you had a permanent base on the moon it seems like Gateway would be pretty irrelevant. Certainly not an important destination. The base/fuel depo would be the hub of activity imo.

perhaps. but i like the idea of having an earth-moon route where every space craft along the way is reusable. i imagine you launching on a 3 stage rocket, first two stages would be recoverable, 3rd stage would also be a reusable launch and re-entry vehicle. it would rendezvous with a translunar shuttle which stays in space (refueling at the gateway). then you have a dedicated ssto lunar surface <-> gateway shuttle. the gateway mostly serves as a fuel depot for the translunar shuttle. when the lunar-gateway shuttle isn't moving personnel it can be topping off the tanks at the gateway (it can operate autonomously or remotely from the moon to save fuel and be able to offload more at the station).

ideally crew transfers would go in both directions simultaneously. the crew launching in the launch-reentry vehicle would trade ships with the crew of the translunar shuttle in earth orbit. and while the translunar shuttle is operating the lunar surface shuttle can make a couple trips to refuel the gateway. you could eliminate the gateway entirely if your lunar surface shuttle could carry both crew in both directions and fuel for the translunar shuttle. thus a large lunar shuttle would be required (making it hard to launch from earth), or multiple trips from a smaller lunar surface shuttle (though making crew wait on board the translunar shuttle for a fuel run). you can also save fuel by using solar-electric propulsion on the trans lunar shuttle, it doesn't have to land so large solar arrays are possible. probably need something like vasimir that can go fast through the van allen belts and do a slow spiral out during the safer parts of the voyage, or an arcjet that can run in pure chem mode for fast burns and save fuel in arc mode.

Edited July 14, 2019 by Nuke

[RuBisCO]

1 hour ago, kerbiloid said:

As the water ice melts at 0°C and boils (in vacuum) at < 100°C, the lunar temperature daily variation is +/- 100°, and the youngest ice on the Moon is 4 bln years old, ULA plans look even more optimistic than such construction in Sahara.

This is mining water from a polar crater that is in permanent darkness via using a mirror to beam light down into it, this is not mining water at the equator or any random place on the moon. 

There is billions of tons of water in permafrost in the dark craters on the poles of the moon. The LCROSS mission found 5.6% water frozen in the soil when it impacted a rocket stage in a shadowed crater on the moon's south pole. 

https://www.researchgate.net/publication/47520015_Detection_of_water_in_the_LCROSS_ejecta_plume

[tater]

I posted a link to ULA plans in another thread, but the take away is that they estimate that delivered from Earth, prop costs are $4000/kg in LEO, $12,000/kf in EML1, and $36,000/kg on the Moon. If they do lunar ISRU, they said $500/kg on the lunar surface, $1000 at EML1, and $3000/kg back at LEO.

If Starship launches cost 125 M$, they can deliver propellant to LEO for $1000/kg, and using ULA's math, they can deliver it to EML1 for $3000/kg. Starship might cost 125 M$/each to build, but with any reuse at all, the cost starts to drop substantially. If they could only get 10X uses, or the equivalent cost (say booster is reused a bunch, and upper stage less or not at all), then they deliver props to LEO for $100/kg, $300/kg to EML1, and $900/kg on the lunar surface. In that case, ISRU is only useful on the Moon. Effectively ISRU becomes useful for getting landers off the Moon. That was close to the old paper I had in the 90s, although at the time they were not assuming water, they were assuming using aluminum oxides as what they were mining.

[RuBisCO]

9 minutes ago, tater said:

I posted a link to ULA plans in another thread, but the take away is that they estimate that delivered from Earth, prop costs are $4000/kg in LEO, $12,000/kf in EML1, and $36,000/kg on the Moon. If they do lunar ISRU, they said $500/kg on the lunar surface, $1000 at EML1, and $3000/kg back at LEO.

If Starship launches cost 125 M$, they can deliver propellant to LEO for $1000/kg, and using ULA's math, they can deliver it to EML1 for $3000/kg. Starship might cost 125 M$/each to build, but with any reuse at all, the cost starts to drop substantially. If they could only get 10X uses, or the equivalent cost (say booster is reused a bunch, and upper stage less or not at all), then they deliver props to LEO for $100/kg, $300/kg to EML1, and $900/kg on the lunar surface. In that case, ISRU is only useful on the Moon. Effectively ISRU becomes useful for getting landers off the Moon. That was close to the old paper I had in the 90s, although at the time they were not assuming water, they were assuming using aluminum oxides as what they were mining.

Starship (I want to go back to calling it BFS  ) has its own propellant economy based on bring liquid methane and oxygen from earth up. A moon based propellant economy would be based Liquid hydrogen and oxygen and be incompatible with Starship. I think Elon has the right idea though, in that the time to develop a moon based LH2/LO2 economy via ULA and Blue Origin, he will have his Earth based LCH4/LO2 economy already running and sending crews to Mars, which is his primary target, not the moon. Considering the infrastructure needed for a moon based propellant economy, Elon is going to have many years maybe even many decades of lead time, there will even be a time where he would be more economical landing on the moon and back, then the competitors, with him landing the equipment his competitors need to build and lunar propellant economy that would only be able to out-compete him in cis-lunar space. 

[tater]

18 minutes ago, RuBisCO said:

Starship (I want to go back to calling it BFS  ) has its own propellant economy based on bring liquid methane and oxygen from earth up. A moon based propellant economy would be based Liquid hydrogen and oxygen and be incompatible with Starship.

Starship could bring water to LEO as dead weight cargo, and it is still cheaper than ISRU if reusable stage 2 spacecraft are a thing. It changes all the economics entirely.

18 minutes ago, RuBisCO said:

I think Elon has the right idea though, in that the time to develop a moon based LH2/LO2 economy via ULA and Blue Origin, he will have his Earth based LCH4/LO2 economy already running and sending crews to Mars, which is his primary target, not the moon. Considering the infrastructure needed for a moon based propellant economy, Elon is going to have many years maybe even many decades of lead time, there will even be a time where he would be more economical landing on the moon and back, then the competitors, with him landing the equipment his competitors need to build and lunar propellant economy that would only be able to out-compete him in cis-lunar space. 

If Starship works, yeah, there's not need for a hydrolox economy, except for the fact that hydrolox is simple better as propellant from an I_sp standpoint. The question is if 80s of specific impulse is worth the alternate storage and boiloff issues. Seems like the best idea would be to drag some NEO to EML1 that has volatiles, then do ISRU there with no gravity well to speak of.

[RuBisCO]

20 minutes ago, tater said:

Starship could bring water to LEO as dead weight cargo, and it is still cheaper than ISRU if reusable stage 2 spacecraft are a thing. It changes all the economics entirely.

If Starship works, yeah, there's not need for a hydrolox economy, except for the fact that hydrolox is simple better as propellant from an I_sp standpoint. The question is if 80s of specific impulse is worth the alternate storage and boiloff issues. Seems like the best idea would be to drag some NEO to EML1 that has volatiles, then do ISRU there with no gravity well to speak of.

I agree on asteroid mining being better then lunar mining. The only problem is that the delta-v is much lower at the sacrifice of many year launch windows from NEO to/from earth. Moving a large enough NEO to earth to be viable as a fuel source would require an absurd amount of propellant. 

Lets assume a NEO that weighs 1 million tons (~100 m wide C-type asteroid), lets assume we use plasma-oxygen electric thrusters (using waste oxygen from refining that asteroid) with an Isp of ~1500 s, to move this asteroid just 50 m/s and perhaps swing by the moon to achieve an lunar gravity assist capture into high earth orbit,  would require 3400 tons of propellant. 

Better would be to mine it at site via automated/AI mining and have it manufacture return ships, ideally solar sails out of cheap aluminium/magnesium refined as co-product of extracting rare metals, that can bring back those rare metals back to earth for profit, or solar electric propelled hauler that can bring thousands of ton of water and methane for a space economy.  Solar sails require no propellant and can be reused and go just about anywhere in the inner solar system if given enough time, to Mars for orbital mirrors, etc, but are limited in that they can't carry much, which is fine if each is moving a few dozen tons of gold-platinum to earth in an entry capsule, but water is going to be many orders of magnitude bigger haul. 

[Ol’ Musky Boi]

On 7/12/2019 at 2:07 AM, Bill Phil said:

I think it only really works well if you have electric launch capability to deliver payloads to lunar orbit from the lunar surface. 

One idea I’ve read about is to bring hydrogen from Earth and acquire oxygen from the lunar surface. Also energy intensive, but it’s not too bad. With electric launch infrastructure you could just deliver raw regolith that’s been compressed to a standard density. Then the oxygen can be removed from the regolith, and if we wanted to we could get metals and some other stuff from the regolith as well which could be beneficial for space infrastructure.

Of course you need an electric launcher on the Moon. And that’s not easy to do.

I wonder how well a slingatron would work on the Moon...?

Getting oxygen from the aluminium oxide in regolith instead of water is a lot more energy intensive. It takes 285.8 kJ/mol to split water, but 1675.5 kJ/mol to split Al₂O₃. I reckon you'd be better off extracting the water and shooting that up instead, at least if you're only trying to build a fuel depot and aren't doing any space manufacturing. 

[Bill Phil]

27 minutes ago, Ol’ Musky Boi said:

Getting oxygen from the aluminium oxide in regolith instead of water is a lot more energy intensive. It takes 285.8 kJ/mol to split water, but 1675.5 kJ/mol to split Al₂O₃. I reckon you'd be better off extracting the water and shooting that up instead, at least if you're only trying to build a fuel depot and aren't doing any space manufacturing. 

Yeah but the thing is there’s plenty of available energy to use, provided you have enough collection area. 

[kerbiloid]

48 minutes ago, Bill Phil said:

Yeah but the thing is there’s plenty of available energy to use, provided you have enough collection area.  

The area to be covered by solar panels and connected with wires brought from the Earth.

Also which way to decompose this aluminium oxide into aluminium and oxide?

Edited July 14, 2019 by kerbiloid